Vito - hissing from the dashboard

[Brinyan]

I have a 2008 Vito Dualiner. For the past few weeks I’ve noticed a loud hissing noise when starting & after switching off the engine. It’s coming from within the dash. A couple of days ago , there was a short spurt of cloudy gas coming from the forward facing air vents. I opened the windows & it soon stopped.
Air conditioning still blowing cold.
Any ideas of what’s going on?
Thanks